Predistortion linearizer for microwave power amplifiers

ABSTRACT

A predistortion linearizer for microwave power amplifiers wherein a single transistor, e.g. of the GaAsFET type, is subpolarized near the pinch-off condition, and carries out the functions both of a gain expander amplifier for recovery of amplitude distortion of the power amplifier and as a command signal generator for a dephaser element for recovery of the phase distortion of the power amplifier.

BACKGROUND OF THE INVENTION

The present invention relates to improvements in microwave poweramplifiers.

Modulations presently in use in radio links, essentially QuadratureAmplitude Modulation (QAM), impose very stringent linearity requirementsfor the radio frequency power amplifier of the transmitter on whichdepends not a little the degradation of the modulated signal.

The power output from the final amplifier devices must be considerablylower than their saturation power so that the nonlinear distortionsthereof introduced satisfy the specifications of the transmitter. Thesedistortions are due to compression of gain at high power found in thetrend of the Amplitude Modulation (AM/AM) distortion curve at high powerand the amplitude modulation/phase modulation (AM/PM) conversion curve,again at high power.

Obviating these distortions usually involves oversizing these finalamplifiers and hence high cost of the power amplifying section.

As known in itself, use of a linearizing network in the transmittingsection permits use of power devices with lower saturation for a givendistortion produced with a resulting increase of efficiency, e.g. forapplication in the transmitters of on-board repeaters in satellitecommunication systems or, for a given saturation power of final devices,such use also allows higher linearity of the amplifier, e.g. forapplications in transmitters for earth stations in such satellitecommunication systems.

A linearization technique presently well known is termed "feed forwarderror control", and includes all the linearizers which use an auxiliarymicrowave amplifier which amplifies an error signal obtained bydetermining the difference between the input signal and the distortedone appropriately attenuated at the output from the main amplifier. Theerror signal is proportional to the distortions generated by the mainamplifier so that, again added with appropriate phase and amplitude atthe output of the main amplifier, this error signal reduces thedistortions affecting the output signal.

It is clear that the merit figure or degree of quality of thislinearization system depends almost exclusively on the balancing of thefinal adder or coupler which subtracts the error signal from the outputsignal of the amplifier. A balancing regulation circuitry (in amplitudeand phase) of this coupler is therefore necessary and is quite complex.In addition, it is a true amplifier-linearizer complex in itself, not anaddition to improve a known amplifier.

Another present linearization technique calls for the use of RF (radiofrequency) predistorters, i.e. nonlinear networks inserted upstream ofthe final microwave amplifier, which distort the input signal by meansof networks embodied with components which work in a nonlinear state inorder to compensate for the AM/AM distortion curve and theamplitude/phase conversion curve AM/PM of the final power amplifier, andwhich guarantee better linearity of the transmitting section. The maindrawback of these known predistorters consists, however, of theexcessive complexity of said networks, and thus of their still excessivecost.

An example of a predistortion linearizer for microwave power amplifiersis described in Italian patent application No. 19497-A/87 filed by thesame applicant Feb. 26, 1987, and incorporated herein.

In this previous Italian patent application there is described apredistortion linearizer applicable upstream of the power amplifier andwhich has a main network including a phase modulator and an amplitudemodulator arranged in cascade. A secondary network with a base bandfrequency is provided. It includes means for amplitude detecting andfiltering part of the input signal so as to produce a detected signalwhich is a function of the instantaneous input power. A pair ofadjustable gain amplifiers are supplied with the detected signal and acton the modulators in such a manner as to give them nonlinear responsecurves, such as to compensate independently for both amplitude and phasenonlinearity of the power amplifier.

Such a linearizer, although it has all the advantages compared with theknown art listed in the application, is still a costly and cumbersomeembodiment, principally because of the presence therein of derivedbranches.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to overcome the abovedrawbacks and to provide a predistortion linearizer for microwave poweramplifiers which is extremely simplified in its circuit structure.Indeed it comprises essentially a single transistor which carries outthe functions both of the gain expander amplifier for recovery of theamplitude distortion of the power amplifier and as a generator of acommand signal for a phase shift element for recovery of the phasedistortion of the power amplifier. In a first form of the embodiment,this single transistor is placed upstream of the final power amplifierand is followed by the phase shift element. In a second form of theembodiment, its function is fulfilled by the same final power amplifier.

According to the invention, a first subpolarized transistor meansprovides gain expansion with increase in power of a signal at its input.A phase shifter means controlled by a continuous component of voltagepresent at an output of the first transistor means introduces a phasedistortion for offsetting the phase distortion of the final poweramplifier.

Further objects and advantages of the present invention will be madeclear by the following detailed description of an embodiment thereof andthe annexed drawings given for purely nonlimiting explanatory purposes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit diagram of a first example of a realization ofthe linearizer which is an object of the present invention;

FIG. 1A shows a bipolar transistor which can replace the FET FT1 shownin FIG. 1;

FIGS. 2, 3, 4, and 5 show the trends of some characteristic parametersof the linearizer and final power amplifier as a function of the powerPi of the input signal;

FIG. 6 shows a first embodiment variation of the phase shift element D1of FIG. 1;

FIG. 7 shows a second embodiment variation of the phase shift element,and

FIG. 7A shows a bipolar transistor which can replace the FET FT1 shownin FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a circuit embodiment of the linearizer which is an objectof the invention and is applied upstream of a power amplifier for thepurpose of compensating for the amplitude and phase distortions. Indeed,the amplitude and phase response curves of a microwave power amplifierare typically as shown in FIG. 2 wherein is shown the qualitative shapeof the output power Pu (AM/AM distortion curve) and the input-outputphase variation VF (AM/PM conversion curve) as a function of input powerPi. The linearizer thus has a dual purpose. First, it expands its owngain at the Pi values so that in FIG. 2 the knee of the curve Pu occursto compensate for said knee and continue the linear shape of pu even inthe immediate proximity of saturation. This latter zone is indicated bya horizontal shape which, however, cannot be offset. The second purposeis to vary its own input-output phase in a manner opposite to the shapeof VF in FIG. 2.

In FIG. 1, RFin, RFout indicate the input and output connectorsrespectively of a radiofrequency signal.

C1, C2 . . . C5 indicate capacitances and L1, L2 inductances of a knowntype.

FT1 indicates a GaAsFET transistor (with gallium arsenide field effect)equipped as known with three connectors, namely a source S, drain D, andgate G1, used in a common source configuration.

R1 indicates a polarization resistance of FT1.

D1 indicates a varactor diode.

RA1, RA2, RA3 indicate common impedance matching networks for input,interstage, and output, respectively, and embodied, for example, in amicrostrip.

VG, VP indicate the supply voltages of the gate of FT1 and of the anodeof D1, and V+ indicates a fixed positive.

C1 and C2 are used as blocking capacitances of the continuous componentat the input RFin and the output RFout, respectively. C4 and thenetworks C3-L2, C5-L1 form low-pass decoupling filters between thesignal and the supplies VG, VP, and V+. C3, C4, and C5 have a groundedend.

The input signal is applied to the gate of FT1 through C1 and RA1. Thevoltage Vg is also applied to the gate of FT1 through the filter C3-L2.The source of FT1 is connected to ground, while the voltage V+ isbrought to the drain through the filter C5-L1 and the resistance R1. Thecathode of the diode D1 is also connected continuously to the drain ofFT1 through the network RA2, and to the output RFout through the networkRA3 and C2.

The transistor FT1 amplifies the RF signal applied at the input RFin.The values of fixed supply voltage V+, resistance R1, and voltage VG arechosen in such a manner as to keep FT1 in underpolarization conditions,i.e. with the working point near the pinch-off region (low values ofdrain-source current Ids and gate-source voltage Vgs).

In this manner an increase in the power of the Rf input signal iscapable of changing the working point of the device, i.e. increasing thecontinuous component of the current Ids. Since the gain of FT1 dependson the continuous component of Ids, the increase of the latter causes anincrease in gain and supplies the desired expansion effect.

FIG. 3 shows the qualitative shape of the gain curve Gi obtained as afunction of the power Pi of the signal RF at the input RFin. By varyingthe parameters of the polarization network of FT1, i.e. VG, R1 and V+,it is possible to obtain a shape of Gi optimized from the point of viewof compensation of the AM/AM distortion curve. The figures also show asa function of Pi the corresponding qualitative shape of the continuouscomponent of Ids.

Due to the presence of the resistance R1, as the continuous component ofIds increases, the continuous component of the drain-source voltage Vdsof FT1 decreases, and hence that of the polarization voltage VL of thevaractor diode D1 (indeed D1 is connected continuously to the drain ofFT1).

Thus, a detected voltage of the modulating signal proportional to thecontinuous component of Ids is localized and is used as a control signalof the phase variation introduced by the downstream device which, in thenonlimiting example described, is the varactor diode D1.

The effect obtained is thus a modulation of the voltage VL which, as isknown, influences the internal capacitance value of the varactor andhence the phase shift introduced therefrom onto the RF signal. Thiseffect can be used with advantage to offset the conversion curve AM/PM.

FIG. 4 shows the qualitative shape of the phase variation FL as afunction of the power Pi of the input signal. The shape of FL can bemade specular in relation to that of VF (shown with broken line in FIG.4) by appropriate sizing of the parameters which influence it, i.e. R1and VP.

To sum up, the transistor FT1 fulfills a dual function: that of gainexpander to offset the AM/AM distortion curve and that of detector of amodulation voltage which directly pilots a phase shifter to offset theAM/PM conversion curve.

More specifically, the gain expansion characteristic is adjusted byvarying VG, and the phase characteristic is adjusted by varying VP.

The device of FIG. 1 follows very rapidly the dynamics of the inputsignal RF, i.e. its response as a gain expander and phase shifter is awide band for the modulating signal because it consists of a signal RFbranch. This is in effect very important because it makes the deviceusable even when the multicarrier modulating signal is wide band, e.g.in transponders for satellites.

In addition, there is an undoubted advantage in terms of size and costreduction of the components of the linearizer which can be embodied withmicrostrip or Microwave Integrated Circuit (MIC) technology using bothdiscrete packaged components and chip-and-wire or integrated inMonolithic MIC (MMIC) technology.

Numerous variations on the embodiment described as an example in FIG. 1are possible, without going beyond the scope of the innovativeprinciples contained in the invention concepts herein.

The linearization function can be performed by the final power stagecomprised, for example, of a transistor of the FT1 type of FIG. 1,brought to a similar working condition.

This may be explained by a digital example also with reference to FIG. 5which shows the shapes of the output power Pu of the final power stagein two working conditions, normal (Ids=2A) and modified in accordancewith the invention (Ids-0.7A), respectively, and which also shows theshape of the continuous component Ids of the drain current of thetransistor of the final stage, as a function of the input power Pi.

Assume a final power stage comprised of a GaAsFET transistor which inlinear conditions has a gain of 10 dB with a working point of Fds=10 V,Ids=2A, and an output saturation power Pusat=40 dBm at modulated signalfrequencies of approximately 6-7 GHz. Under these conditions, the shapeof output power Pu as a function of input power Pi is as indicated inFIG. 5 by parameter Ids=2A, with the knee zone to be linearized.

If we now decrease the gate-source voltage Vgs of the transistor toobtain Ids=0.7A, the gain of the linear zone decreases to approximately7 dB. Under these conditions when the input power Pi increases, Ids alsotends to increase to a nominal value of 2A in conformity with the shapeof Ids shown in the figure. The increase of Ids causes an increase inthe gain Gi of the transistor just in the zone where the curvePu(Ids=2A) had the knee, introducing in this case a self-linearizationeffect: the amplitude in the knee zone of the curve Pu(Ids=0.7A)decreases greatly, offsetting the distortion curve AM/AM.

As concerns offsetting the conversion curve AM/PM, the phase variationscan be offset by a circuit of known type upstream and which is pilotedby the voltage Vds of the final transistor as described with referenceto FIG. 1.

The variation now described can be used to advantage in QAM multi-levelmodulation systems where the RF output power is not constant, but variesconsiderably in relation to the average value depending on the point ofthe constellation of symbols to be transmitted. For example, in the64QAM system there is a difference of approximately 8 dB between averageand maximum RF output power. There is a considerable current savings;for the low levels of the constellation for which the output power Pu islow, the current Ids stays around 0.7A and reaches 2A only for thehigher levels for which Pu is high. Since the points of theconstellation are all equally probable, the average Ids current will beapproximately 1.1-1.2A, and not 2A, with a clear consumption savings. Adecrease of gain in the linear zone from 10 to 7 dB does not involveproblems because the 3 dB difference is necessary only at the powerpeaks.

A second variation calls for the embodiment of the phase shifter as inFIG. 6, wherein the same symbols as in FIG. 1 indicate the samecomponents interconnected in the same manner.

In FIG. 6 there has been added a second varicap diode D2 equal andantiparallel to D1 for the signals. The anode of D2 is connected to thecathode of D1, while the cathode of D2 is polarized by the directvoltage VP2 and is connected to ground for the signals through thefilter capacitance C6.

The embodiment of FIG. 6 serves, in case it is desired, to offset aphase variation which may be either leading or delaying. One diodeoffsets the delaying phase variations and the other the leading ones. Byappropriately sizing the voltages VP and VP2, it is possible to make oneof the two diodes work while excluding the other, thus obtaining aleading phase shift with D2 inserted, or delaying with D1 inserted.

A third variation calls for an embodiment of the phase shifter as inFIG. 7 wherein the same symbols as in FIG. 1 indicate the samecomponents interconnected in the same manner and performing the samefunctions.

In FIG. 7 the phase shifter is embodied by a GaAsFET transistorindicated by FT2, and is polarized in such a manner as to function as anormal amplifier in a common source configuration with the continuouscomponent of the drain-source current Ids2=1/2Idss. The latter is themaximum value of the drain current for Vgs=0.

R2 indicates the load resistance of FT2.

C7, C8, and C9 indicate capacitances and L3, L4 indicate inductances ofknown type. C9 and the networks C7-L3, C8-L4 form low-pass decouplingfilters between signals and supplies for output RFout, gate FT2, anddrain FT2; C7, C8 and C9 have one end grounded.

RA4 indicates an output matching network with the same function as RA1,RA2, and RA3.

By varying the continuous component of the gate-source voltage of FT2,there is obtained a variation of the continuous component of the draincurrent Ids. This does not cause an appreciable variation in the gain ofFT2, since FT2 is in a linear operation condition, but does vary thevalue of gate-source capacitance Cgs of FT2. The effect obtained isequivalent to that supplied by the varactor diode D1 of FIG. 1, i.e.variation of the phase FL of the output signal RFout which offsets theAM/PM conversion curve (see FIG. 4).

In this case also the voltage detected in the modulation of themodulating signal (which is proportional to the continuous component ofIds of FT1 located at the ends of R1) is used as the control signal ofthe phase variation introduced by FT2. To bring this control signal toFT2, it is necessary to continuously pair the two transistors FT1 andFT2, bringing to the input of FT2 the voltage taken from R1 either inphase or in phase inverted fashion, depending on whether the phasevariation desired is leading or delaying, respectively.

To introduce a phase delay there can be used the component indicated byAMP in FIG. 7, which is a continuous inverting amplifier with variablegain and a pass band having a width at least double that of themodulating input signal band. AMP receives and amplifies the continuouscomponent of the drain voltage of FT1 at one end of R1, and supplies itwith a changed sign to the gate of FT2 through the filter C7-L3 toobtain a modulation effect of the continuous component of the gatevoltage of FT2. This effect is translated into a variation of thecapacitance Cgs of FT2.

AMP can be embodied by a reversing stage of any known type. If embodiedby a GaAsFET transistor, the entire circuit of FIG. 7 can be integratedin MMIC technology in a simple and economical embodiment.

If FT2 is not to lead but is to delay in phase, the amplifier AMP willhave the same characteristics as above, except that it will not bereversing.

As another variation, the transistors called for in the various forms ofembodiment described can also be of another type, e.g. bipolar.

This is shown in FIGS. 1A and 7A, wherein the bipolar transistorsreplace the FET FT1.

Although various minor changes and modifications might be proposed bythose skilled in the art, it will be understood that we wish to includewithin the claims of the patent warranted hereon all such changes andmodifications as reasonably come within our contribution to the art.

We claim as our invention:
 1. A predistortion linearizer for a finalmicrowave power amplifier subject to gain compression and phasedistortion, comprising:first subpolarized transistor means for providinggain expansion with increase in power of a signal at its input; biasingmeans at said same input of the first transistor means for setting asubpolarization operating point thereof; and phase shifter meanscontrolled by a continuous component of voltage present at an output ofsaid first transistor means for introducing a phase distortion foroffsetting the phase distortion of said final power amplifier.
 2. Alinearizer according to claim 1 wherein said first transistor means isplaced upstream of said final power amplifier.
 3. A linearizer accordingto claim 2 wherein said first transistor means comprises a bipolarcommon emitter configuration whose base polarization voltage isadjustable to obtain said subpolarization.
 4. A linearizer according toclaim 2 wherein said phase shifter means comprises a varactor diodeplaced in parallel with said first transistor means by being coupledcontinuously to the output of said first transistor means and signalspresent there, a polarization voltage of said varactor diode beingadjustable.
 5. A linearizer according to claim 2 wherein said phaseshifter means comprises an amplifier stage placed downstream of saidfirst transistor means and comprises a transistor in a common sourceconfiguration whose input is coupled continuously with the output ofsaid first transistor means.
 6. A linearizer according to claim 5wherein said continuous coupling between the first transistor means andsaid transistor comprises a wide band inverting continuous amplifier. 7.A linearizer according to claim 5 wherein said continuous couplingbetween the first transistor means and said transistor comprises a wideband noninverting continuous amplifier.
 8. A linearizer according toclaim 2 wherein said phase shifter means comprises an amplifier stageplaced downstream of said first transistor means and comprises a bipolartransistor in a common emitter configuration whose input is coupledcontinuously with the output of said first transistor means.
 9. Apredistortion linearizer for a final microwave power amplifier subjectto gain compression and phase distortion, comprising:first subpolarizedtransistor means placed upstream of said final power amplifier forproviding gain expansion with increase in power of a signal at itsinput; phase shifter means controlled by a continuous component ofvoltage present at an output of said first transistor means forintroducing a phase distortion for offsetting the phase distortion ofsaid final power amplifier; and said first transistor means comprising aGaAsFET in a common source configuration and whose gate polarizationvoltage is adjustable to obtain said subpolarization.
 10. Apredistortion linearizer for a final microwave power amplifier subjectto gain compression and phase distortion, comprising:first subpolarizedtransistor means placed upstream of said final power amplifier forproviding gain expansion with increase in power of a signal at itsinput; phase shifter means controlled by a continuous component ofvoltage present at an output of said first transistor means forintroducing a phase distortion for offsetting the phase distortion ofsaid final power amplifier; and said phase shifter means comprising twovaractor diodes placed in antiparallel and coupled continuously to theoutput of said first transistor means and signals present there, apolarization voltage of each of said diodes being independentlyadjustable.
 11. A predistortion linearizer system, comprising:a finalmicrowave power amplifier having a given gain compression and phasedistortion; upstream of an input of said final power amplifier asubpolarized transistor means for providing gain expansion with increasein power of a signal at its input for compensating said given gaincompression of the final microwave power amplifier; subpolarizingbiasing means connected at an input of said subpolarized transistormeans for setting said subpolarized transistor means in a subpolarizedcondition wherein a working point is near a cutoff point of thetransistor means such that said increase in power of the signal at theinput of the subpolarized transistor means changes the working point ofthe device further away from said cutoff point in order to increase gainthereof to provide said gain expansion; and phase shifter means which isindependent of said subpolarizing biasing means and which is controlledby a component of voltage present at an output of said subpolarizedtransistor means for introducing a phase distortion for offsetting saidgiven phase distortion of said final power amplifier.
 12. A finalmicrowave power amplifier having gain and phase compensation tocompensate for a given gain compression and phase distortion of theamplifier, comprising:a microwave power amplifier transistor; means forproviding gain expansion with increase in power of a signal at an inputof the amplifier transistor for compensating said given gain compressionof the power amplifier; subpolarizing biasing means connected at aninput of said amplifier transistor for setting said amplifier transistorin a subpolarized condition wherein a working point is near a cutoffpoint thereof such that said increase in power of the signal at theinput of the transistor changes the working point of the device furtheraway from said cutoff point in order to increase gain thereof to providesaid gain expansion; and phase shifter means which is independent ofsaid subpolarizing biasing means and which is controlled by a voltagepresent at an output of the amplifier transistor for introducing a phasedistortion for offsetting said given phase distortion of said finalpower amplifier.
 13. A final microwave power amplifier havingcompensation for gain compression and phase distortion thereof,comprising:a transistor amplifier; subpolarization biasing meansconnected to an input of said transistor amplifier for providing gainexpansion with increase in power of the signal at the input of thetransistor amplifier by setting a subpolarization operating pointthereof; and phase shifter means controlled by a continuous component ofvoltage present at an output of said amplifier transistor forintroducing a phase distortion for offsetting the phase distortion ofsaid final power amplifier.
 14. A predistortion linearizer for a finalmicrowave power amplifier subject to gain compression and phasedistortion, comprising:first subpolarized transistor means for providinggain expansion with increase in power of a signal at its input; andphase shifter means controlled by a continuous component of voltagepresent at an output of said first transistor means for introducing aphase distortion for offsetting the phase distortion of said final poweramplifier.
 15. A final microwave power amplifier having compensation forgain compression and phase distortion thereof, comprising:a subpolarizedtransistor amplifier; subpolarization biasing means connected to saidtransistor amplifier for providing gain expansion with increase in powerof the signal at the input of the transistor amplifier by setting asubpolarization operating point thereof; and phase shifter meanscontrolled by a continuous component of voltage present at an output ofsaid amplifier transistor for introducing a phase distortion foroffsetting the phase distortion of said final power amplifier.